X-ray CCDs have become the mainstream technology for space-based X-ray imaging spectroscopy as a result of the exceptionally high quality of the devices currently in production. As the need for more information from space-based X-ray imaging spectroscopy increases, there has been a trend for ever-larger CCD arrays. This has led to the development of very large CCD arrays like the MIT Lincoln Laboratory CCID-20 (2048×4098 pixels). However, the continuation of this trend towards even larger CCD arrays is threatened by a number of technological factors.
The most important factor is the rapid increase in power consumption on a system level as the number of pixels and size of the sensor arrays grow larger. This power consumption consists of two main components: 1) power required for driving large parallel gate capacitances and 2) the power consumed in signal processing circuits.
In addition, large modem CCD arrays used in space-based X-ray imaging spectroscopy are more vulnerable to radiation damage because charge has to be transferred over very large distances (>5 cm) in silicon. Also, the large CCD array sizes lead to slower frame times, and therefore, degrade the time resolution of the integrated instrument.
One conventional device developed to address the various technological factors discussed above is the CMOS Active Pixel Sensor. Although CMOS Active Pixel Sensor characteristic features are: 1) extremely low power consumption; 2) random access; 3) extensive on chip signal processing; and 4) intrapixel signal processing; CMOS Active Pixel Sensors are not readily applicable to X-ray detection.
First, CMOS Active Pixel Sensors are implemented using CMOS technology where transistors are made in relatively heavily doped wells. This results in an extremely shallow depletion depth on the order of 0.2-0.5 μm, thereby rendering the CMOS Active Pixel Sensors useless for X-ray spectroscopy. The shallow depletion depth could be addressed by increasing the substrate resistivity that in turn would dramatically increase the CMOS Active Pixel Sensors' sensitivity to single-event upsets and latch-ups. However, this increase in sensitivity would present other problems when utilizing CMOS Active Pixel Sensors in the radiation environment of a space borne instrument.
Second, the fill factor of existing CMOS Active Pixel Sensor designs is unacceptably low. This is especially true for the ever more popular CMOS Active Pixel Sensor designs with complicated pixel structure which are being used in order to improve some particular characteristics of a sensor, such as readout or flat pattern noise.
Third, the typical readout noise of CMOS Active Pixel Sensors is currently at least an order of magnitude higher than that of CCDs. High readout noise makes the current CMOS Active Pixel Sensor technology unacceptable for X-ray detection. Although a CMOS Active Pixel Sensor has been developed with a readout noise of 4.5 electrons, this CMOS Active Pixel Sensor works in the so called “soft reset” mode which implies a very slow recovery of the output node voltage to the reset level. This very slow recovery of the output node voltage to the reset level is unacceptable for the continuous readout mode required for X-ray sensor operation.
Fourth, every pixel of CMOS Active Pixel Sensor has its own amplifier with a different gain and offset. Individually calibrating millions of pixels is extremely difficult, both experimentally and computationally, and secular variations in these calibrations will remain as unknown systematic errors in the data. CCDs have many fewer amplifiers, so accurate calibration is possible.
In X-ray astronomy, raw calibration data is used to determine unknown parameters of a detector model. To do so, the penetration of X-rays into the device and the collection of the resulting photoelectric charge must be modeled. Constructing an adequate model is not easy for a CCD, but it is far more difficult for a CMOS Active Pixel Sensor because the CMOS Active Pixel Sensor pixel structure is far more complex.
Moreover, conventional X-ray CCD sensors suffer from: 1) low pixel rates, which cause pileup in high throughout imaging spectrometers; 2) high detector cost, which negatively impacts the affordability of large focal planes; 3) near cryogenic temperature operation, which negatively impacts thermal system cost and the risks in ground testing; 4) high power consumption, which negatively impacts the cost of the satellite and the viewing angle constraints; and 5) dimensionally large pixels, which require relatively long focal length optics and negatively impact polarization sensitivity.
Therefore, it is desirable to design an X-ray CCD array device that can operate at high speeds and at low power (less than that of conventional devices). It is also desirable to design an X-ray CCD array device that can operate with near unity quantum efficiency and exhibit low readout noise. Moreover, it is desirable to achieve these operational parameters at near room temperature (˜10° C.), rather than requiring cryogenic temperatures (T˜−90° C. or less).
Furthermore, it is desirable to design an X-ray CCD array device that is optimized for measurements of X-ray polarization, for application as focal planes for short focal length optics, high-resolution X-ray telescopes, and for extremely large format coded-aperture CCD cameras.